There are a number of internal power consuming prosthetic devices now employed or contemplated for implantation in the human body. A common problem with all of these devices is providing an effective, safe power supply. Smaller devices such as pacemakers can use replaceable batteries. The necessity of surgically replacing such batteries periodically is not a significant problem. With respect to devices requiring more power than the pacemaker, battery-supplied power is inadequate. For example, devices such as artificial hearts require up to 20 watts of continuous power. A battery designed to provide such power for 60 days would be very large. Nuclear power supplies are also inappropriate since the shielding requirements would make these unsuitable. Further, if the shielding failed, the results could be detrimental to the user's health.
All devices used in humans for mechanical circulatory assistance have, therefore, required a permanent opening in the skin for energy transfer. These include pneumatic conduits for balloon pumps, the Jarvik total artificial heart, blood conduits for the Thoratec (Pierce Donachy) and Abiomed sacular pumps, an electrical cable for the Novacor solenoid pump, the Thermo Cardio Systems, Inc. (TCI) assist device, and a spinning torque cable for the Nimbus Hemo pump intravascular turbine. Whereas these have occasionally been used for extended periods of time without infection (over two years in one Jarvik patient and over one year in current Novacor, Thoratec and TCI recipients), clinical and experimental observations indicate that such an integumental break presents a continuing risk of infection.
Principally for this reason, none of these devices except the Jarvik have been seriously proposed for long term circulatory support. All the others have been promoted as strictly temporary aids for use during expected cardiac recovery or during the waiting period for a transplant donor. Infections have been minimal for these short periods.
Electrical induction has long been entertained as a means of delivering power from extra corporeal source across intact integument. In 1961, a transformer operating with radio frequency alternating current from an external to a subcutaneous coil was reported by Schuder, Stephenson and Townsend. It was reasoned that a coil within a coil configuration could be more efficient and a tube pedicled skin nap was utilized. Within this tube of skin (shaped like a suitcase handle and attached to the chest wall at either end) lay a secondary coil while the primary coil with an iron core (allowing a lower frequency current to be used), surrounded it. Efficiencies of 97% (57 watts, 20 kHz) were reported.
Two groups of investigators have pursued these concepts for the past decade and have been developing inductive energy transmission systems seriously intended for powering of clinical circulatory assist devices. A belt skin transformer was developed by LaForge at Novacor which consists of a narrow single turn flexible secondary coil implanted in the subcutaneous tissue around the waist and a five turn extra corporeal primary coil worn in a belt. This has effectively transmitted 15 watts of continuous power at more than 75% efficiency in both in vitro models and experimental animal models. This system is intended to be coupled with a modified version of Novacor's current temporary solenoid operated intra corporeal left ventricular assist system to form a support system for long term use. Animals with the implanted device have survived for over two years with little difficulty reported.
An induction device has been developed by Thermedics Inc. (now TCI) which is situated in and on the anterior abdominal wall. The implanted secondary coil is made of 16 turns of braided copper wire wrapped around a dome-shaped polyurethane appliance within the abdominal subcutaneous tissue. The primary is a 3 turn coil in a ring that is worn surrounding the mound produced by the secondary appliance and secured by a belt. Transmission of 24 watts has been demonstrated. In efficiency studies in animals this has delivered 6 to 12 watts of usable power with a 3 wall loss (65%-70% efficiency). Most of the losses were demonstrated in external components and about 1 watt was lost in the transformer itself, presumably as heat. A clinical form of this device is to be used with an electric version of Thermo Medical System's current pneumatic left ventricular assist system. These devices seem likely to offer a practical means for extra- to intra-corporeal energy transfer. Consideration of their use in patients, however, suggests some possible problems.
The only known prior use of a life supporting device that had to be maintained in the surface position was the radio frequency induction coil used in pacemakers before introduction of satisfactory implantable batteries. These worked very well electrically (the very low power requirements of pacemakers needed a much less efficient inductor than assist devices) but there was a high fatality rate clinically due to inadvertent displacement by the patient. Further changes in electrical load of a pumping device or minor component failure in the activating circuit can potentially increase the heat produced in implanted secondary coils. Potential for dissipation of this heat without damage to surrounding tissue is limited by tissue blood supply. A serious burn of the tissue layer separating the primary and secondary coils could lead to device infection. Further, the discomfort and annoyance of a device that the patient can constantly feel in contact with his or her skin compounded by the psychological impact of knowing that the itch, tickle, or irritation is to be there for life, is impossible to anticipate or calculate.
Treating heart failure mechanically requires power. While the net mechanical energy to pump five liters blood per minute at 100 mmHg incremental pressure, a typical requirement for an adult human at rest, is just 1.10 Watt [5 L/min * 100 mmHg * (0.001 m.sup.3 /L) * 1 min/60 s * (133 {N/m.sup.2 }/1 mmHg) * 1 W/(1 Nm/s)=1.10 W], desired reserve capacity and achievable efficiency make a supply of 10 to 20 watts preferable for either total artificial hearts or ventricular assist devices. Although internal sources (nuclear cells, chemical batteries, chemical fuel cells or harnessed skeletal muscles) are attractive, each has limitations not yet resolved. External power remains a requirement for devices doing most or all of the circulatory work. That power has been delivered in many ways.
Whether pneumatic tubes, hydraulic tubes, electric wires, or sheathed torque cables, direct connections are simple, reliable, and mechanically secure. Their limitation is that they may also be routes for infection of implanted hardware and contiguous tissue. Although meticulous entry site care and careful cable design have reduced this risk for some systems to a point that is perhaps tolerable for the time spent awaiting a heart transplant donor, some driveline infections still occur with systemic consequences (J. Heart and Long Transpl. 15:S73, 1996). Further, this experience is in a very controlled, usually in-hospital environment. Extending this protection to the five year, ten year, or longer survival likely needed to make "permanent" circulatory support a seriously acceptable offering, especially in a more relaxed, "normal" lifestyle and environment, may be a severe challenge. The somewhat analogous home maintenance of externalized dialysis shunts has succeeded in brief applications, but these have only rarely remained totally infection-free for many years in outpatients. An artificial heart line cannot be so simply removed and inserted elsewhere while eradicating episodic sepsis. External lines are one, though not the only, potential source of intrathoracic blood pump infections, and the prognosis of these infections is not good (J. Cardiovasc. and Thor Surg. 98:506-9, 1989). We believe this justifies continued interest in energy transmission through intact integument.
As discussed above, electrical induction has been considered by many investigators to be a reasonable means for doing this. Schuder, Stephenson, and Townsend (Trans. Society Artificial Internal Organs 1961;7:327-329) in 1961 reported air-core transformers operating with radio frequency current from an external to a subcutaneous coil. Both Andren, et al. (The Institute of Electrical and Electronics Engineers: Trans. of Biomed. Eng. 1968; 15: 278-280) and Newgard et al. (Hegyeli R., ed. Proceedings of the First Artificial Heart Program Conference. Washington, DC: US Government Printing Office, 1969:927-936) reasoned that a coil-within-a-coil concentric configuration could be more efficient and reported devices using a tube-pedicled skin flap. Within this tube of skin attached to the chest wall at either end lay a secondary coil, while the primary coil and an iron core (allowing use of lower frequency power) surrounded it. Myers et al. reported external and subcutaneous coils, each with a ferrite core giving total weight of 16 ounces and working at low audio frequencies (Trans. Am. Soc. Artificial Internal Organs 1968;14: 210-214). Other investigators have continued to pursue and develop these concepts. LaForge and colleagues at Novacor, Inc. (now Novacor Division of Baxter Healthcare Corp.) have developed a 400 to 600 kHz "belt skin transformer." (In: Andrade, J. D., ed. Proceedings of the Internation Symposium on Artificial Organs, Biomedical Engineering and Transplantation. New York: VCH Publishers, 1987: 95-107). This consists of a narrow single-turn flexible secondary coil implanted in the subcutaneous tissue around the waist and a five-turn extra corporeal primary coil worn in a belt. This has effectively transmitted 15 watts of continuous power at more than 75% total system efficiency in both in vitro and experimental animal models. The system is intended to be coupled with a modified version of Novacor's solenoid-operated intra corporeal left ventricular assist system for long-term support. A clinical system will include both external and internal storage batteries. Sherman, Dasse, and associates at Thermedics, Inc. (Thermo Cardiosystems, Inc., TCI) have developed a 180 kHz induction device to be placed in and on the anterior abdominal wall (Trans. Society Artif. Internal Organs 1981;27: 137-139). The implanted secondary coil is 16 turns of braided copper wire in a dome-shaped polyurethane appliance within the abdominal wall. Lying on the skin around the mound produced by the secondary is a ring containing a 3 turn primary. It has transmitted up to 24 watts of power. In chronic animal studies, it delivered 6 to 12 watts of usable power with 3 watts total loss and about 1 watt loss in the coils themselves (65% to 70% total system and about 90% coil-to-coil efficiency). This is intended to power the electric TCI intra corporeal left ventricular assist device.
A number of other devices share this external primary ring and subcutaneous secondary cone coil arrangement. That arrangement was developed by Schima and associates at the University of Vienna functions at a considerably higher frequency (1.0 MHZ), permitting greater freedom of displacement with satisfactory maintenance of coupling (Proceedings of the International Workshop on Rotary Blood Pumps. Vienna 1991: 77-81). The Ottawa group uses an "autotuned" system in which frequency varies in the 400 to 500 kHz range depending on coil separation (Artif. Org. 17:940-7, 1993) while the Penn State system operates at 160 kHz (ASAIO Journal. 39:M177-84, 1993). Other recently described devices operate at 210 kHz (Artificial Organs 18: 80-92,1994), 230 kHz (IEEE Transaction on Magnetics. 29:3334-6, 1993),240 kHz (ASAIO Journal 39:M208-12, 1993). Most of these have dome-shaped implanted secondary appliances with thicknesses of 1.5 to 3 cm and diameters similar to the 7.1 cm of Penn State or the 6.6 cm of the Ottawa device. The secondary appliance described by Ahn et al. is notably smaller, only 3.8 cm in diameter. Primary rings are usually 2 to 3 cm greater in diameter than secondary ones.
Most of these transformer devices function well, in that they have been shown to effectively transfer sufficient energy to supply expected needs for practical electric artificial hearts and assist devices. They may well afford more lifestyle freedom than do the care regimens mandated by skin-penetrating, direct connection lines, even if the safety of such regimens were to be demonstrated for indefinite periods. This assumes that neither maintenance of alignment, tissue warming, nor magnetic flux leakage become serious clinical problems. The constant tissue warming, while causing problems in early prototypes of at least one device, exceeded surrounding tissue by only 1.6 to 2.5.degree. C. in others and, for some sort of physical position maintenance (other than the brief grade periods granted by internal batters) may be annoying, but not likely tolerable.
Melvin, U.S. Pat. No. 5,109,843, discloses the first extra- to intra-corporeal power supply positioned within a defunctionalized intestinal pouch. Specifically, an ileal pouch is used. The disclosure of this reference is incorporated herein by reference in its entirety. This employs a single cylindrical or bulbous ferrite core with primary wrappings around the core in the pouch. Secondary windings extend around the outside of the pouch, i.e. within the body. Due to the design and construction of the ferrite core, it suffers from significant magnetic flux leakage.
Magnetic flux leakage may impose significant lifestyle restrictions of its own. Air-core power transformers operating at the radio frequencies required for reasonable coupling also generate substantial flux fields well beyond the subject's body. That invites interference, both to external electronic devices and from contiguous metal and magnetic materials. Interference with electronic devices might be only a nuisance, though considering the power levels involved and the ubiquity of such devices in most people's lives, it could be a major nuisance. Interference to the device from contiguous magnetic or even nonmagnetic metal could be far more serious. This was tested in only one of the publications reviewed. The Ottawa device showed a 10% attenuation of function with a "metal object" (mass and type of metal not given) in contact with the primary coil. Rigorous testing with progressively more massive metal of varying magnetic permeabilities has not been reported. The consequence of the hospital-liberated, device-dependent person resting upon or leaning against all manner of common metal structures, some quite massive and of both magnetic and non-magnetic metals, is an open question. There are at least theoretical grounds to expect, in the absence of absolutely perfect coil alignment, that such everyday activities as sitting in wrought iron chairs, leaning on structural steel pillars or car doors, and walking by fire hydrants or bank safes may not be innocuous. Some degree of transmitted power attenuation might be addressed by compensatory design adjustments; a battery-draining near short-circuit through the contiguous metal could be far more serious. For a technology whose rationale is largely based on safely extending its recipients' range of activities and environments, this is no small concern. Clearly, safety testing to quantify or disprove such risks is indicated.